25 C.F.R. § 151.3 : Indians — BUREAU OF INDIAN AFFAIRS, DEPARTMENT OF THE INTERIOR — LAND AND WATER — LAND ACQUISITIONS — Land acquisition policy

Land not held in trust or restricted status may only be acquired for an individual Indian or a tribe in trust status when such acquisition is authorized by an act of Congress. No acquisition of land in trust status, including a transfer of land already held in trust or restricted status, shall be valid unless the acquisition is approved by the Secretary.

(a) Subject to the provisions contained in the acts of Congress which authorize land acquisitions, land may be acquired for a tribe in trust status:
(1) When the property is located within the exterior boundaries of the tribe's reservation or adjacent thereto, or within a tribal consolidation area; or
(2) When the tribe already owns an interest in the land; or
(3) When the Secretary determines that the acquisition of the land is necessary to facilitate tribal self-determination, economic development, or Indian housing.
(b) Subject to the provisions contained in the acts of Congress which authorize land acquisitions or holding land in trust or restricted status, land may be acquired for an individual Indian in trust status:
(1) When the land is located within the exterior boundaries of an Indian reservation, or adjacent thereto; or
(2) When the land is already in trust or restricted status.

25 C.F.R. § 151.3